Inhibition of substance P release from spinal cord tissue after pretreatment with capsaicin does not mediate the antinociceptive effect of capsaicin in adult mice

Substance P (SP) is released from primary afferent fibers in response to nociceptive stimuli. Capsaicin, which produces an initial hyperalgesic response followed by persistent antinociception, also elicits release of SP from primary afferent fibers. Capsaicin pretreatment decreases the content and release of SP from primary afferent fibers. This effect on SP has been hypothesized to mediate the antinociceptive effect of capsaicin. To test this hypothesis, mice were injected intrathecally (i.t.) with antinociceptive doses of capsaicin or SP(1–7) before superfusion of spinal cord tissue with 3 μM capsaicin 24, 48, 96 or 168 h later. N-terminal metabolic fragments of SP that accumulate after capsaicin-induced SP release and are involved in the antinociceptive effect of capsaicin, were also tested. Like capsaicin SP(1–3), SP(1–4) and SP(1–7) were each antinociceptive when injected 24 h before nociceptive testing. However, at this time there was no decrease in capsaicin-evoked release of SP in tissue from capsaicin- and SP(1–7)-pretreated animals compared to those injected with vehicle. In contrast, capsaicin-evoked SP release decreased significantly in tissue from mice pretreated with capsaicin or SP(1–7) 48 h prior to testing. d-Substance P(1–7), which prevents antinociception, blocked capsaicin- and SP(1–7)-induced decreases in SP release, indicating that these effects are mediated by SP N-terminal activity. Total spinal cord content of SP did not differ amongst treatment groups. These data indicate that antinociception does not appear to depend on decreases in SP release or content as antinociception precedes decreases in SP release.